The long-term objective of this proposal is to use DNA sequence data to examine the hypothesis that enzyme polymorphisms, associated with specific amino acid substitutions, are under the influence of selection. Specifically, the study will examine patterns of DNA sequence polymorphism at two enzyme loci, glucose-6-phosphate dehydrogenase (G6PD) and glycerol- 3-phosphate dehydrogenase (GPDH), in Drosophila melanogaster. These loci have been the targets of a significant number of functional biochemical and population genetic studies for decades, and both loci posses simple allozyme polymorphisms. Primary DNA sequences have been published for both genes. It is proposed to sequence specific regions in 40 copies of each gene sampled from North America and Europe. Sequencing this large number of genes will be facilitated by using the polymerase chain reaction (PCR) to amplify targeted sequences from genomic DNA preparations. This essential methodology has been successfully acquired by the PI's laboratory, and has been used to sequence two copies of the G6PD gene. Ten copies of the same gene regions will also be sequenced in Drosophila simulans. This species possesses no allozyme polymorphisms for these two loci. Statistical tests formulated from recent population genetic theory will be applied to contrast the patterns of polymorphism and sequence divergence between species. The results of these tests will determine if excessive levels of DNA polymorphism are found in the regions linked to the amino acid substitutions responsible for the allozyme polymorphisms. Significantly increased levels of DNA sequence polymorphism are expected if the allozyme polymorphisms are maintained by balancing selection. This study is important because sizable programs are underway, or proposed, to systematically map and sequence large parts of the genomes of several species including man. The quantity of DNA sequence data is expected to increase exponentially in the next decade. One feature certain to emerge from those studies will be heterogeneity in levels of DNA sequence polymorphism across regions of the genome. An important task of theoretical and experimental population genetics will be to interpret this heterogeneity in the context of neutral theory and natural selection. This proposal is a plan to examine one potential feature or cause of this variation.